Revisiting salicylidene-based anion receptors

Area of Science:

• Analytical Chemistry
• Organic Chemistry
• Supramolecular Chemistry

Background:

• Salicylidene-based compounds are utilized as colorimetric and fluorimetric anion sensors.
• The stability of imine bonds in these receptors under various conditions is crucial for their sensing applications.

Purpose of the Study:

• To investigate the hydrolysis of imine bonds in salicylidene-based receptors upon interaction with anions.
• To elucidate the mechanism of anion-induced imine bond cleavage using NMR spectroscopy.

Main Methods:

• Proton Nuclear Magnetic Resonance (¹H-NMR) spectroscopy was employed to study the interactions.
• Experiments were conducted in DMSO-d₆ with various salicylidene compounds (SL, CL1, CL2) and quaternary ammonium salts of halides and oxo-anions.
• Control experiments were performed to assess the role of moisture.

Main Results:

• Hydrolysis of imine (-N=CH-) bonds was observed in salicylidene receptors in the presence of halide and oxo-anions.
• The mono-salicylidene compound CL1 exhibited the most significant hydrolysis.
• NMR signals corresponding to salicylaldehyde and the disappearance of imine peaks confirmed the cleavage, alongside evidence of hydrogen bonding and tautomerism.

Conclusions:

• Anion-induced hydrolysis of imine bonds is a significant degradation pathway for salicylidene-based receptors.
• The rate of hydrolysis is influenced by the structure of the salicylidene compound and the presence of anions.
• A general mechanism for anion-induced imine bond hydrolysis in these systems has been proposed, offering insights for designing more robust anion sensors.